1. Field of the Invention
This invention relates to seismic prospecting, used for depicting the presence and geometry of subsurface geological strata. As such, it is widely employed for petroleum exploration, as well as for other purposes. This is classified in the Patent Office under Classes 340/15.5 Ts, 346/14 MR, 346/33 C, and 340/15.5 DP.
2. Description of the Prior Art
It is believed that no technical description of geophysical exploration need be given, in view of the number of modern textbooks that have dealt with this topic. A considerable description of prior art pertinent to this particular field is contained in U.S. Pat. No. 3,806,864, Broding, et al. An earlier patent in this particular field is U.S. Pat. No. 3,283,295 Montgomery.
In both of these patents, it is pointed out that it is possible to record locally at a convenient spot adjacent a seismometer (or more usually a group of seismometers), the ordinary cumbersome cables used in the prior art having been omitted for ease and portability and increased speed of operation. In U.S. Pat. No. 3,806,864, a radio link is used to turn on selectively from a control point (ordinarily the instrument truck) those specific recorders needed out of a larger total number for a given set of record traces. At each recorder, identification and recording parameter data are partly transmitted from the control point and partly supplied by the individual recorder unit to form the "header" information on the tape.
Field personnel acceptance of this kind of recording system depends chiefly upon having available equipment which confirms prior to each shot or set of vibrations that the desired recorders, and only those recorders, are functional and ready to receive and record the seismic signals picked up by the various seismometers (geophones).
In my copending U.S. application given above (Ser. No. 529,308), one arrangement for providing the field operating crew with confidence that operations will be satisfactory is accomplished by providing what can be called a status alarm. This alarm functions at a particular group recorder when it has been energized (for example as pointed out in my prior patent above) and is presumptively ready for operation. This alarm system is actuated only when the major systems within the group recorder are ready to operate. The alarm provides a visual indication or a sound, or both. Thus, any of the field personnel that are reasonably within sight or sound of the individual group recorders can assure himself that those units are in operating condition just prior to the taking of the data.
In copending Application Ser. No. 562,282, of Francis R. Freeman, filed in the U.S. Pat. Office on Mar. 26, 1975, a single flexible cable is provided which is plugged into all the seismometer group recorders, and a separate playback unit at the recording truck is provided for monitoring playback of the seismometer group recorders. Command signals from the central control unit in the truck are transmitted over this circuit (or by radio) to arrange for a playback unit in any chosen group recorder to playback selected portions of the records stored, the information being converted from coded digitized form to analog form for recording at the truck.
The arrangement described in the Freeman application permits assurance after the record has been made that suitable data have been produced and recorded at whatever group recorder is checked upon. However, it is apparent that such a system requires too much time for routine use in confirming the operation of all group recorders involved in a particular seismic recording. Also, it is desirable to have confirmation of operating status of equipment in advance of a recording rather than subsequent to it. While the system shown in my prior application does this locally for the personnel who are near selected group recorders, it certainly cannot accomplish this when the instruments are laid in rugged terrain, or, as frequently happens, when the instrument layout or seismometer spread may occupy a linear distance of the order of a mile or more.
A quite different system for recording at the central recording unit or truck the output of a plurality of seismometers using a single two-wire transmission line has been described in the Philippe Angelle U.S. Pat. No. 3,652,979. Here, the individual units connecting the seismometers to the transmission line are controlled by pulses transmitted on the line from a pulse generator; by coding the pulses appropriately, the signals arrive at the central unit in any desired time sequence. This system, however, does not provide for a status indicator in advance of operation or recording, so the desired confirmation for the operator is unavailable. In fact, I know of no system at the present time which is immediately adaptable to such a purpose except that described in this specification.